This chapter has examined various components in corrosion products formed on the surface of pure iron and a few iron-based alloys, which were analyzed using in-situ X-ray diffraction measurements with synchrotron radiation and in-house X-ray sources. A specially designed cell was used for in-situ diffraction measurements of corrosion products freshly formed on the surface of the iron samples by cyclic exposure to wet and dry atmospheres. Ex-situ X-ray diffraction measurements by synchrotron radiation have also been carried out for a very small amount of sample extracted from wet corrosion products. These results stressed that corrosion products are formed of ferrous ions as well as ferric ions in atmospheric corrosion. Rust layers formed in mild and weathering steel is known to consist primarily of fine α-FeOOH particles, which are almost all ferric ions [3-6], since these corrosion products are likely to be oxidized by air. However, the formation of ferrous ions must occur at the corrosion front, which is the vicinity of the substrate iron, by anodic reaction. The above experiments may simulate the formation of different oxides from such ferrous ions. Although corrosion conditions in this work are not necessarily identical to the case for the protective rust layer formed in the atmospheric cycle, the results show that components of corrosion products are influenced by atmospheric factors. Since several alloying elements are contained in weathering steel, they are incorporated in the rust layer as cations and anions. Thus, a rust layer with a very complicated structure may be formed in the weathering steel, and as a result, a protective membrane acts against water, oxygen and detrimental anions such as chloride ions.